Add Keep-Alive to keep the subscription WebSocket open

**Summary**
Playground fails with an error after playing with subscriptions.

**Version**
Not applicable

**Description**
In step 11. of the Apollo tutorial for Android (1), playing with the Subscription in the Playground (2) stopped working after some time with this error:
```
{
"error": "Could not connect to websocket endpoint wss://apollo-fullstack-tutorial.herokuapp.com/graphql. Please check if the endpoint url is correct."
}
```

(1) https://www.apollographql.com/docs/android/tutorial/11-subscriptions/
(2) https://apollo-fullstack-tutorial.herokuapp.com/

Let me know if you need further information.